1. Field of Invention
The present invention relates to the field of communication system, specifically to a low-noise amplifier (LNA), and more specifically to a wideband active balun LNA topology with narrow-band filtering and noise cancelling.
2. Description of Related Arts
Besides the basic requirements of low-noise amplification and impedance matching, the RF input front end of a wideband receiver is desired to be capable of handling tracking filtering and single-to-differential (S-to-D) conversion with stable second-order linearity and lower power consumption. In order to achieve such requirements, a general approach is as follows: RF signals passing through S-to-D network or LNA, and then being connected to a filter load through a buffer stage and a transconductance (Gm) stage to accomplish narrow-band tracking filtering, and then being connected to a mixer through the buffer stage. The buffer stage and the transconductance (Gm) stage that serve as interface stages consume a substantial sum of power and use more chip-area and also limit the overall linearity and noise performance. The structures that are generally used in wideband LNA are analyzed below.
Traditional wideband low-noise amplifier structures usually have a single-end input and output.
The simplest structure as above described is common-gate (CG) or common-source (CS) single-transistor amplification. The structure with common-source and input parallel resistor matching is very simple, but still commonly used. The disadvantage of such structure is that it is impossible to achieve low noise and impedance matching simultaneously.
Besides single-transistor amplifier and simple feedback amplifier, many wideband low-noise amplifier structures adopt a noise cancellation technology to achieve lower noise figure as well as achieving source impedance matching. For example, F. Bruccoleri published the paper entitled “Wide-Band CMOS Low-Noise Amplifier Exploiting Thermal Noise Cancelling” on IEEE JSSC in 2004, in which the proposed structure implements noise cancellation of the transistor dedicated for impedance matching. Later, a similar structure was applied to a digital TV Tuner chip. For example, Gupta, M. published the paper entitled “A 48 to 860 MHz CMOS Direct-Conversion TV Tuner” on IEEE ISSCC in 2007, in which a similar structure is used. However, the disadvantage of such a structure is poor IIP2. The paper entitled “A Wideband CMOS Low-noise amplifier Employing Noise and IM2 Distortion Cancellation for a Digital TV Tuner” published on IEEE JSSC in 2009 by Donggu Im mentioned a method of CMOS and current mirror, which can achieve noise cancellation and further improve IIP2 of a single-ended input and single-end output low-noise amplifier (LNA) to some extent. However, this structure has some drawbacks as follows: a current mirror limits the bandwidth of the circuit and the current mirror itself introduces much noise whereas second-order linearity basically depends on the matching between PMOS and NMOS, which can result in a relatively sensitive IIP2. The wideband low-noise amplifier structures introduced above are capable of implementing a relatively low noise figure, however, they can't satisfy the requirement of single-to-differential conversion on a chip, and it is difficult for them to attain good second-order linearity either.
However, only a small number of LNA structures can obtain single-to-differential conversion in a wide bandwidth. For example, Blaakmeer, S. C. published the paper entitled “Wideband Balun-LNA With Simultaneous Output Balancing, Noise-Cancelling and Distortion-Cancelling” on IEEE JSSC in 2008, which proposed a single-ended input, differential output and noise-cancelling amplifier (LNA) structure that is capable of cancelling the noise of the common-gate transistor dedicated for input impedance matching. However, in order to obtain a balanced differential voltage output, the output load resistances of two differential signals should be different from each other. Therefore, a load filter circuit similar to LC Tank cannot be directly connected to the output thereof. In view of implementing filtering, a stage such as transconductance stage (Gm) or buffer stage must be inserted between the LNA and the filter load (similar to the foregoing traditional approach), which simultaneously degrades noise performance and linearity performance, thereby further adding extra power consumption. For example, Ru, Z. published the paper entitled “A 300-800 MHz Tunable Filter and Linearized LNA Applied in a Low-Noise Harmonic-Rejection RF-Sampling Receiver” on IEEE JSSC in 2010, which proposed a single-to-differential conversion structure using cascaded inverting CMOS stages. Although the structure can implement single-to-differential conversion on a chip, a filter circuit is placed at an RF input since it can't be implemented at output, resulting in a very narrow input matching bandwidth. Furthermore, this structure can obtain good IIP3 using a post-linearization approach, however the relatively good IIP2 is sensitive, which essentially relies on the matching between PMOS and NMOS. In addition, the noise figure of the entire circuit is relatively high without using noise cancellation technology. For example, Donggu Im published the paper entitled “A CMOS Active Feedback Balun-LNA with High IIP2 for Wideband Digital TV Receivers” on IEEE Transactions on Microwave Theory and Techniques in 2010, which proposed a structure of implementing single-to-differential conversion through differential pair and also implementing wideband matching with a differential-to-single-end feedback. So the structure has very stable IIP2 and a highly symmetrical differential output. The disadvantage is that the output impedance characteristic directly affects the input matching, thus failing to implement low-noise amplification and narrow-band filtering in one stage, if additional transconductance stage is inserted for filtering, the overall noise and linearity performance will be degraded. For example, Pui-In Mak published the paper entitled “A 0.46-mm2 4-dB NF Unified Receiver Front-End for Full-Band Mobile TV in 65-nm CMOS” on IEEE JSSC in 2011, which proposed a noise cancellation circuit based on Nauta's scheme. This circuit can achieve balanced differential output and implement relatively good single-to-differential conversion by turning a part of transconductance from DC connection into AC connection, and inserting a multi-stage differential current balancer (DCB) circuit. Besides, the structure can be directly connected to LC narrow-band filter at output due to the same output load for differential path. The disadvantage of this structure is that: balanced differential output is implemented through a multi-stage DCB circuit, which is therefore constrained by both the voltage headroom of the circuit and the adjustability capability of the DCB circuit. When the DCB circuit is well designed to meet the requirement of differential balancing, the input part of the circuit still encounters the trade-off between a noise figure and impedance matching. In addition, IIP2 performance of the circuit is still sensitive and is easily susceptible to the power supply, load, and performance of DCB circuit. For example, Danilo Manstretta published the paper entitled “A Broadband Low-Power Low-Noise Active Balun with Second-Order Distortion Cancellation” on IEEE JSSC in 2012, which proposed, based on the noise cancellation circuit of Nauta's Group, a single-to-differential conversion structure that increases second-order linearity by adopting a common-mode feedback circuit. Although the structure can implement desirable IIP2 and NF, the unbalanced differential output makes it impossible for a load filter circuit similar to LC Tank to be implemented, unless an additional buffer stage or transconductance (Gm) stage is used.
In previous realization, none of the existing manners for wideband LNA structure is found capable of implementing single-to-differential conversion, narrow-band LC filtering, low noise and stable second-order linearity simultaneously in one stage.